The following image represents the whole system designed in Proteus CAD:
All the connectors on the left side represent inputs and outputs from the Nicla Sense ME, based on the official pinout, moreover, on the middle right side there are the connections required for the I2C communication protocol between the Nicla and Xiao, as well as power.
At the bottom of the figure, the I2C sensors aforementioned, SI1145 and MLX90614, can be found, with the SEN0203 DFRobot Gravity heart rate sensor, all connected to the SCL and SDA bus, with the exception of the SEN0203 which connects directly to Analog A0 from the Nicla Sense ME.
At the top left side of the figure, there is a P mosfet activated voltage divider with a ratio of 1.3, formed by R1 and R4, to have a highly accurate voltage reading from the battery, while having the minimal voltage loss when not in use. The power consumption when disabled is equal to 42 nanoamps, with a stray voltage of 420 microvolts. Meanwhile when the P MOSFET is switched on, the maximum voltage from a 4.2V battery is 3.2V which is right under the allowed 3.3V of the microcontroller, which results in a power usage of 2.1 milliamps. The act pin, can be driven low or high, with the latest enabling the P MOSFET. The 12 kilo-ohms resistor on the P-MOSFET and the 3 kilo-ohms resistor on the NPN BC547 transistor, are used for transistor biasing. A transistor is based in order to make the emitter base junction forward biased and collector base junction reverse biased, so that it maintains in active region, to work as an amplifier.
At the top right side, the DFPlayer Mini can be observed connected to the Nicla UART connections TX and RX, with a 1 kilo-ohm resistor on the TX bus to reduce the noise that gets passed to the speaker. Moreover, a 22 picofarad ceramic non-polarised capacitor helps reduce the popping noise that can be rendered because of a drop in current when the MicroSD card is being read. The DFPlayer Pin 16, represents the BUSY pin which is driven high at 3.34 volts when sleeping, and 0.04 when playing a file, and is used to check whether or not a file has finished playing. The UART communication works with the same voltages.
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